A major goal in evolutionary genetics is to connect genotypic and phenotypic variation both within and between species to better understand the functional consequences of molecular variation, and ultimately the genetic basis of traits that affect fitness. Several male reproductive traits in mammals are likely to play a key role in fitness. Recent work in a variety of species has shown that genes involved in male reproduction are often targets of natural selection. The three main objectives of this study are to: 1) Test the hypothesis that genes involved in male reproduction in house mice are subject to positive natural selection, both within and between species. Rates of divergence in candidate loci will be investigated among four closely related Mus lineages and Rattus rattus. Polymorphism will be investigated in a sample of 20 wild-derived M. domesticus sampled from its native range; 2) Document naturally occurring phenotypic variation in sperm competitive ability using these same 20 M. domesticus males. Males will compete against a common male genotype for fertilizations; 3) Connect phenotypic variation in sperm competitive ability to genotypic variation observed in candidate loci. These studies will help uncover the genetic basis of male reproductive fitness in a natural population of house mice. [unreadable] [unreadable]